introduction
The full-drive off-road vehicle is extremely harsh and the load changes is relatively large, how to apply frequent changes in driving resistance and terrestrial adhesion, is a key issue that improves different drivers of all-drive off-road vehicles, and manipulating convenience. In order to solve the above problems, matching automatic gearbox is one of the best choices for solving problems.
The automatic transmission mainly includes the following four types:
1) AMT mechanical automatic transmission is to install electronic selection mechanism on the basis of a manual transmission to achieve a gear automatic switching. It can mitigate the driver's labor intensity, but there is a power interruption in the shift, and the shift is low. Mainly used in low-end market passenger cars and commercial vehicles.
2) The AT liquid mechanical automatic transmission is composed of a liquid torque converter, a clutch, planetary gear mechanism, shifting mechanism, and the like. With the development of electronic technology, comprehensive performance is further improved, especially in heavy vehicles and pass The high-driven off-road vehicle vehicle has a more advantageous advantage of the high-driven, and the application of AT is more widely used [2].
3) The speed ratio of the CVT continuously gearbox is a series of continuous values, which has good economic and comfort, but the torque of the steel strip is limited, and only small displacement engines can only match small displacements.
4) The DCT dual clutch automatic transmission has the advantages of high transmission efficiency, and the power transmission of the shift process is not interrupted, and the domestic passenger cars match the matching rate. DCT includes dry and wettation, dry clutches are prone to overheating problems, which cannot be applied to severe vehicles in a harsh environment.
Like the clutch's AMT system and manual gearbox, it is shifting power source and shifts, and has a big impact on the power to the wheels, and the smoothness of shifting is difficult to compare with AT [3].
In this paper, the feasibility analysis argumentation of the matching AT automatic transmission is performed on the driving conditions of the full drive off-road vehicle.
1 clutch transmission system analysis
1.1 Hydraulic torque converter
Hydraulic torque converter is mounted between engine and planetary transmission mechanism
Improve the increase and speed transfer of engine torque, mainly including pump wheel, turbine and guide wheel, see Figure 1.
Figure 1 Schematic diagram of working principle of liquid torque converter
Due to the liquid torque work member relying on liquid contact, power transmission gears can prevent the transmission system from overloading and effectively reducing starting and shifting impact. At the same time, the ortholaking changing characteristics of the speed and torque can be achieved within a certain range, and the starting performance and passability of the vehicle are improved. When the vehicle's travel resistance is relatively large, the clutch is in unlocking state, and the superiority of the liquid torque converter is used to improve the off-road capability [4].
The rate of liquid torque converters is equal to the ratio of the turbine speed and the pump wheel speed, and the calculation formula is shown (1):
(1) in the formula:
The torque ratio of the liquid torque converter is equal to the ratio of the turbine torque and the pump wheel torque, and the calculation formula is shown (2):
(2) In:
A model of liquid torque converter is not shown in Figure 2 [5].
Figure 2 Curve of the liquid torque converter
As can be seen from Fig. 2, full-driven off-road vehicles are at a low speed start condition, and the torque converter acts on the engine output torque, smooth through the obstacle, in line with the use of off-road vehicles.
The AMT gearbox does not have a liquid torque converter that cannot achieve an increase in the engine output torque, and the mechanical member is a rigid connection, and the vibration of the engine output shaft cannot be eliminated.
1.2 clutch
The full-drive off-road vehicle is complex, the clutch is high, and the AMT clutch is mainly composed of an active portion, the driven portion, the compression mechanism, and the manipulation mechanism, see Figure 3. When the clutch is required, the actuator separates the clutch friction with the flywheel so that the torque of the engine cannot be passed to the gearbox input shaft. In contrast, when the clutch is required, the diaphragm spring passes the clutch friction plate and the flywheel to transfer torque, thereby driving the engine.
Figure 3 AMT friction clutch pressure plate and driven disc
In order to avoid damage to the clutch due to long-term friction, the current clutch load is calculated based on the rotational speed of the clutch and the sliding time, and the load limit value is exceeded. The clutch protection algorithm is shown in Figure 4.
Figure 4 Schematic diagram of the clutch protection algorithm
When the clutch is overloaded, there is the following protection measures:
1) The clutch torque can be gradually increased to the clutch fully engaged to avoid the clutch to continue to bear the load. This approach allows the vehicle to reduce the engine speed to the engine speed, and if necessary, the engine is turned off;
2) The start stop can be allowed according to the calculated clutch load limit;
3) According to the boundary conditions applied, automatic offensive can be performed during start or peristaltic travel;
4) If the clutch slip is still not eliminated even when the clutch has been engaged, the engine torque will be limited by the gearbox control device in a manner intervention.
In summary, after trigger the clutch protection function, the vehicle partial function is limited, and the power is lowered.
The national standard has a minimum stable vehicle speed of no more than 3km / h on the full drive, and the full drive off-road vehicle operating condition is much creep, and the AMT transmission is a dry clutch, close to natural air-cooled heat, at low speed halfway In the extreme condition, the clutch is frequently smooth, the clutch is slim, the clutch temperature is rapidly warming, and the clutch protection is easily triggered, and the damage to the clutch hardware is irreversible.
The AT gearbox has a hydraulic torque converter and a plurality of wet clutches, even at low speed and extreme harsh conditions, increase the rapid reduction of the clutch lubrication flow to quickly reduce the clutch temperature, so the AT transmission is more suitable for matching full drive off-road vehicles.
2 shifting quality
High quality shifts are smooth, fast, no noise, and the degree of wear due to the device is below the allowable value. The reason why the AT transmission is widely used is because it satisfies the above 4 points.
The AMT gearbox mechanical structure determines that the shift project must separate the clutch, resulting in power interruption, power interruption time comprises partial clutch separation, picking, selection, synchronization, hanging, and partially engaging the clutch. After the clutch is separated to the slip point and the clutch is bonded to the slippoint, the clutch can deliver a portion or all torque, but the output torque of the engine has decreased, so this part of the time is not completely powered, but can be equivalent to partial time. Power interruption. During the picking, selection, synchronization, and hosted, the power is completely unable to pass, so it is an absolute power interruption time. Therefore, the entire shift process vehicle experiences acceleration, sliding, and then accelerates three stages, which affects driving comfort and affects the dynamics of vehicle. Especially during the acceleration process and the feeling of loss of power is more pronounced during the traveling process.
A heavy commercial vehicle AMT shift process is shown in Figure 5, the entire shift process 1.85 seconds, absolute power interruption time 375 milliseconds, 1.475 seconds are part of the time power interruption.
Figure 5 AMT shifting process
In particular, when the ramp is driving, the increase in the restriction of the slope is particularly rapidly decreased. See Figure 6.
Figure 6 AMT ramp shifting process
The AT gearbox is quickly and smooth. The exchange of automatic gearbox input torque The re-allocation only occurs between the separated clutch and the combined clutch, and the torque transmitted in the short time shift process is unchanged, and the power is not interrupted. The vehicle is acting as driving. Comfortable and dynamic is better [6]. The principle of the power boost control is shown in Figure 7.
Figure 7 AT transmission dynamic upgrade control schematic
3 conclusions
Aiming at full-drive off-road vehicle complex and diverse driving conditions, the feasibility of matching the AT automatic gearbox is analyzed, and the following conclusions are obtained:
1) The AT automatic transmission is matched with a liquid torque converter. When the vehicle travel resistance is relatively large, the clutch is in unlocking state, and the engine output torque can be enlarged, and the off-road ability is enhanced, and AMT Do not have this advantage.
2) All drive off-road vehicle off-road and low speed creep conditions, the clutch is slim, and if there is no timely cooling, damage the clutch hardware and irreversible. The AT transmission has a hydraulic torque converter and a wet multi-chip clutch. The clutch cooling effect is good, and AMT is a dry clutch. In frequent starting conditions, due to the low speed, the heat exchange heat effect is poor, it is easy to trigger the clutch thermal protection Even burn the clutch, affect the vehicle function.
3) The full-driven ride car has a high power requirements, and the AT transmission is a non-warrant interrupt shift, driving comfort and power performance. When the AMT gearbox shifts, the clutch must be separated, resulting in the power interruption of the vehicle, and even the magnitude of the slope, due to the increase in the resistance of the ramp, the speed is lower than the shift, poor dynamic.
In summary, the full-driven window is more suitable for matching the AT automatic gearbox.
